MAINT: statistics review: remove unused functions in stats._support

Closes gh-708
Closes gh-710
Closes gh-711
Closes gh-712
MAINT: rewrite and test stats.itemfreq.  Complete removal of stats._support

Closes gh-599 (stats.itemfreq statistics review)
Closes gh-706 (stats.abut statistics review - removed)
Closes gh-707 (stats.unique statistics review - removed)
Merge pull request #2778 from rgommers/stats-cleanup

Clean up stats._support, close statistics review issues